Capacity Calculator

When developing circuits with high frequency bus systems like DDR4, it is very important to ensure the signal integrity by controlling the impedance of paired bus tracks.

PCB-Investigator offers a simple possibility to calculate the capacity between those bus tracks and your ground signal.

By selecting your impedance controlled net (or only a part of it) and the corresponding ground net, PCB-Investigator calculates all overlapping regions between them and applies the formula for plate capacitor for all single overlapping areas. The stack-up information must be entered before to apply the correct layer distances.

Paired impedance controlled nets like high frequency buses or clock signals should have the same capacity to ground to ensure the signal integrity.

One of the most common reasons for different capacity and therefore bad signal integrity is caused by via openings in your ground plane. When one of the bus signals is touching or crossing those via openings, the track is no more completely above the ground plane as intended. Even a small overlapping can lead to significant difference in the capacity compared to the other bus tracks. This easily has a bad influence on your signal integrity (see images below).